Intermolecular Forces of Attraction - Week 3, Makati High School

Summary

This document is a module from Makati High School on intermolecular forces. It describes the different types of intermolecular forces, including their relative strength, examples, and importance in various phenomena. The document also includes essential questions and learning objectives.

Full Transcript

Department of Education MAKATI HIGH SCHOOL Gen. Luna St. Brgy. Poblacion, Makati City Week 3 – Module 3 INTERMOLECULAR FORCES OF ATTRACTION Essential Questions 1. How do molecules attract each other? 2. What are the different forces of attractio...

Department of Education MAKATI HIGH SCHOOL Gen. Luna St. Brgy. Poblacion, Makati City Week 3 – Module 3 INTERMOLECULAR FORCES OF ATTRACTION Essential Questions 1. How do molecules attract each other? 2. What are the different forces of attraction between molecules? Have you ever seen insects walking on the surface of the water? In nature, there are attractive and repulsive forces. Some of these attractive forces also exist among very small particles of matter. These enable small insects, like the water strider in the photo on the right, to walk on water. These attractive forces are also responsible for other phenomena such as the round shape of a water droplet or the physical states of substances at room temperature. Without these attractive forces, water may not exist as a liquid at 25 °C or boil at 100 °C. Can you imagine what will happen if water is a gas at room temperature? Such is the importance of these attractive forces that there is a need for us to understand them and learn how they affect the things around us. Learning Objective Describe the general types of intermolecular forces (S11/12PS-IIIc-d-17) Types of Molecular Attraction Intermolecular forces are attractive forces present in between molecules. Although there are many, four types of intermolecular forces of attraction (IMFA) are most commonly observed. These are London dispersion forces, dipole-dipole forces, ion-dipole forces and hydrogen bonding forces. The first two are collectively known as van der Waals forces of attraction​, named after the Dutch scientist Johannes van der Waals. 1. London Dispersion Forces (LDFs) These are the weakest type of IMFA and are present in between all electrically neutral molecules ― polar and nonpolar molecules. This IMFA was named after the German-American physicist Fritz London who proposed the origin of this intermolecular attractions. A momentary dipole or instantaneous dipole occurs due to uneven electron distributions in neighboring molecules as they approach one another. Thus, the more electrons that are present in the molecule, the stronger the dispersion forces will be. LDFs are the only type of intermolecular force operating between non-polar molecules. Since this force is weak, attraction can be easily broken down. That is why, nonpolar substances have lower boiling point and melting point. 2. Dipole-Dipole Forces These are attractive forces that are a moderately strong type of IMFA and are present in between polar molecules. Dipole-dipole forces are the result of the electrical interactions among dipoles on neighboring molecules. This means that the partially positive end of one molecule interacts with the partially negative end of a neighboring molecule. For example, HCl is a polar molecule. It has partially positive and partially negative ends. Dipole- dipole force exists between the partially positive end of one HCl molecule and the partially negative end of another HCl molecule. 3. Ion-Dipole Forces This result from the electrostatic attraction of a molecule containing a dipole and an ion. The strength of this kind of IMFA increases as the charge of the ion increases. It is often observed in solutions such as brine (NaCl in water). When NaCl dissolves in water, it exists as Na​+ and Cl​-. The cation Na​+ is attracted to the partially negative O atom of water while the anion Cl​- is attracted to the partially positive H atom of water. 4. Hydrogen Bonding It is a special kind of dipole-dipole force and one of the strongest type of IMFA. Hydrogen bonding is an attractive force that exists when hydrogen is bonded to F, O, or N. In such cases, the partially positive hydrogen of one molecule interacts with the partially negative F, O, or N atoms in another molecule. This relatively strong attraction explains why molecules with this type of IMFA tend to have high boiling and melting points. Many unusual properties of water are attributed to hydrogen bonding. Consider the water molecule, H​2O. The hydrogen of one molecule is attracted to the oxygen atom of another molecule. Formaldehyde does not exhibit hydrogen bonding on its own. However, in the presence of water, the O atom of formaldehyde can form hydrogen bonds with the H atoms of water. Formaldehyde becomes a hydrogen bond acceptor while water is the hydrogen bond donor. A hydrogen bond donor is the molecule that provides the hydrogen atom participating in a hydrogen bond while a hydrogen bond acceptor is the molecule that contains the lone pair-bearing electronegative atom. Essential Questions 1. What are the different properties of substances affected by intermolecular forces of attraction? 2. Where can these applications on the effects of intermolecular forces of attraction seen in real life context? Learning Objective 2. Explain the effect of intermolecular forces on the properties of substances (S11/12PS-IIId-e-19) Liquids have some similarities with solids and gases such as high densities and their ability to resist compression. These properties can be attributed to the intermolecular forces of attraction which hold the molecules together in order to occupy a volume of a certain space of a container. Surface tension is the tendency of a fluid to acquire the least possible surface area. Imagine a small drop of any liquid. It is assumed that a drop is spherical in shape. This is because of the intermolecular forces of attraction present in the molecules of liquids. Cohesive forces are attractive forces that exist between molecules of the same kind while adhesive forces are attractive forces that exist between the molecules of different types, such as a liquid and a solid surface. Molecules with stronger intermolecular forces will exert greater cohesive forces and acquire less surface area than those with weaker IMFA. This means that liquids with strong IMFA have high surface tensions. In contrast, liquids with weaker IMFA have low surface tension. Viscosity is the measure of a fluid’s resistance to flow. Liquids that flow easily have low viscosity while liquids that do not flow readily have high viscosity. Molecules with stronger intermolecular forces have greater resistance to flow because it is difficult for the molecules to move and slide past one another. Thus, they have higher viscosity compared to those with weaker IMFA. Viscosity is also affected by temperature. When the temperature is high, the molecules have more energy, and it becomes easier to overcome the IMFA. This leads to lower viscosity. Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid phase in a closed system. Molecules with stronger intermolecular forces have less tendency to escape as gas, and thus, lower vapor pressure compared to those with weaker IMFA. Water, which exhibits hydrogen bonding, has lower vapor pressure compared to acetone, CH​3COCH​3. Since more energy is needed to disrupt the IMFA in water to convert it to vapor, its boiling point will also be higher than acetone. In fact, the boiling point of acetone is only 56 °C. Volatility is the tendency of a liquid to evaporate. Liquids that evaporate easily are also called ​volatile​ liquids. Acetone is an example of a volatile liquid.

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